need help with sounds

[woly]

hi guys i recently tried to use a line in cable to make my mp3 to ply through my mac speakers, but now when ever i plug headphones/speakers or anything in the headphone slot nothing works, the only thing that works is music coming through the mac speakers, i have downloaded line in software and soundcore to change the outputs/inputs but nothing seems to work so if anyone could help it would be much appreciated
cheers
woly x